1. Field of the Invention
The present invention relates to a pump-turbine and more particularly to a method of controlling the opening and closing of wicket gates of a pump-turbine.
Generally, various components such as runner of a pump-turbine, particularly of a high-head type, are designed to achieve sufficient centrifugal pump action so as to obtain the high-head during pump operation. However, this design badly affects the turbine operation of the pump-turbine. When the performance of the pump-turbine designed in this manner is shown by the performance curve representing the relationship between speed per unit head (N.sub.1) and discharge per unit head (Q.sub.1) under a predetermined opening of wicket gates, this curve includes in the turbine operation area a first section where the value of Q.sub.1 reduces with the increase of the value of N.sub.1 and a second section where the value of Q.sub.1 reduces with the decrease of the value of N.sub.1. For convenience of explanation, the second section will be referred in this specification to "S-section". Further, the pump-turbine performance in the S-section will be hereinafter referred to "S-performance". During the turbine operation in the S-section, the value of torque per unit head (T.sub.1) also is reduced as the value of speed power unit head (N.sub.1 ) decreases.
Ordinarily, the turbine operation of the pump-turbine is effected in the abovementioned first section. However, in the case where the speed per unit head (N.sub.1) is suddenly increased because of for example a removal or loss of load carried on the pump-turbine, the pump-turbine initiates to operate in the S-section. When the operation in the S-section is initiated, the discharge per unit head (Q.sub.1) and speed per unit head (N.sub.1) are first reduced tracing the S-section from one end to the other, and thereafter Q.sub.1 and N.sub.1 are increased tracing the S-section in the opposite direction. This reciprocal tracing on the S-section is repeated nearly endlessly without taking particular measures. Also the torque per unit head (T.sub.1) is repeatedly reduced and increased during this pump-turbine operation. It is desired that the pump-turbine operation in the S-section is avoided, since this operation causes in the upstream penstock and the downstream draft tube as well as in the pump-turbine an abnormal hydraulic pressure variation inclusive of large pressure rise and drop with the resultant severe water hammer and even water column separation in an extreme case. It is to be noted that the abovementioned removal of load would occur, when the generator driven by the pump-turbine loses its load due to a failure or burning out of a transformer or such failure of remote facilities on the same transmission line, and that the water hammer is very severe when the penstock or draft tube, or both, is long.
2. Description of the Prior Art
With the ordinary structure, the wicket gates of the pump-turbine are controlled by a governor to rapidly close upon sudden removal of load from the pump-turbine. However, if the rapid closure of the wicket gates is effected and thus the discharge per unit head (Q.sub.1) is reduced when the pump-turbine operates tracing the S-section in the Q.sub.1 -reducing direction, the above-discussed disadvantageous pressure variation will be accelerated. There have been proposed various wicket gate controlling methods intending to overcome such disadvantage by controlling the closure of the wicket gates upon sudden removal of load. However, these methods do not assure that the rapid closure of the wicket gates is effected always at an appropriate timing. It is considered that the prior art methods have been proposed without sufficiently analyzing and understanding the S-performance of the pump-turbine.